1. Field of the Invention
The present invention relates to an optical fiber transmission line. More specifically, the invention relates to an optical fiber transmission line provided with an optical gain equalizer, and the optical gain equalizer thereof.
2. Description of the Related Art
In order to transmit a signal light through a long distance, an optical wavelength multiplex transmission system comprises an optical fiber amplifier which is arranged in an optical repeater on an optical fiber transmission path to directly amplify the signal light. The optical fiber amplifier includes a rare earth doped optical fiber, an excitation light source, and an optical coupler for supplying a light from the excitation light source to the rare earth doped optical fiber. As an amplification characteristic (gain) of the optical fiber amplifier depends on a wavelength of an incident signal light, many optical gain equalizers and optical gain equalizing methods have been presented to compensate for such wavelength dependency [e.g., Unexamined Patent Publication Laid-Open No. 4 (1992)-269726, Unexamined Patent Publication Laid-Open No. 6 (1994)-276154, Unexamined Patent Publication No. 8 (1996)-223136, Unexamined Patent Publication No. 3 (1991)-44206]. Usually, the optical repeater includes an optical fiber amplifier and an optical gain equalizer. A gain characteristic of such an optical fiber amplifier in a signal wavelength band is represented by an upward convex curve with respect to a wavelength of a signal light. In other words, a gain in a middle wavelength in the signal wavelength band is large. On the other hand, a loss characteristic of the optical gain equalizer with respect to an optical wavelength is represented by a downward convex curve with respect to a wavelength of a signal light. In other words, a loss in a middle wavelength in the signal wavelength band is large. The gain characteristic of the optical fiber amplifier and the loss characteristic of the optical gain equalizer cause a gain characteristic of the optical repeater in the signal wavelength band to be flat.
The optical wavelength multiplex transmission system comprises a plurality of optical repeaters, which are arranged at specified distances on the optical fiber transmission line for connecting an optical transmission equipment and an optical receiving equipment with each other. In the case of the optical repeater provided with the optical gain equalizer, a gain with respect to a signal light of the optical power set at the time of designing is flat. Accordingly, optical signals of respective channels of a wavelength multiplexed signal light are propagated through the optical fiber transmission line, each having an optical power equal to one another.
In the actual optical multiplex transmission system, however, losses on the optical fiber transmission line are not uniform but varied widely. Thus, there is large variance of optical power among signal lights entering the respective optical repeaters. On the other hand, when a signal light of an optical power different from a design value enters the optical repeater, a gain of the optical repeater in the signal wavelength band is monotonously reduced or increased with respect to a wavelength of the signal light. Specifically, if a power of a signal light entering the optical repeater is smaller than a specified optical power, a gain of the optical repeater is monotonously reduced from a short wavelength to a long wavelength. Conversely, if a power of an signal light entering the optical repeater is larger than the specified optical power, a gain of the optical repeater is monotonously increased from a short wavelength to a long wavelength.
If a loss on the optical fiber transmission line is larger than the design value, an input power of a signal light entering the optical repeater is smaller than the design value. Consequently, a wavelength gain of the optical repeater shows a characteristic of a monotonous reduction to a long wavelength side in the wavelength band, and a power of a signal light having a shortest wavelength is larger than an optical power of a signal light having a longest wavelength. Similarly, if a loss on the optical fiber transmission line is smaller than the design value, an input power of a signal light entering the optical repeater is larger than the design value. Consequently, the wavelength gain of the optical repeater shows a characteristic of a monotonous increase to a long wavelength side in the signal wavelength band, and a power of a signal light having a shortest wavelength is smaller than an optical power of a signal light having a longest wavelength.
Thus, even if the optical gain equalizer is installed in the optical repeater to compensate for the gain characteristic of the light amplifier, it is difficult to equalize gains in the entire optical multiplex transmission system.
An object of the present invention is to equalize gains on an entire optical transmission line so as to provide an equal power for each of signal lights, even if an optical power of a signal light entering an optical repeater is deviated from a design value because of loss variance or the like of an optical fiber.
In order to achieve the foregoing object, an optical gain equalizer of the present invention has a characteristic of a monotonous change of loss in a signal wavelength band. This loss may be divided into characteristics of a monotonous reduction from a short wavelength to a long wavelength, and a monotonous increase from a short wavelength to a long wavelength. Specifically, the optical gain equalizer may be composed of a well-known optical element such as an optical etalon filter, an optical band-pass filter, an optical fiber grating or the like. An optical fiber transmission line of the present invention includes such optical gain equalizers arranged at specified intervals. An optical wavelength multiplex transmission system of the present invention comprises an optical transmission equipment for sending out a wavelength multiplexed signal light, an optical receiving equipment for receiving the wavelength multiplexed signal light by separating a wavelength of the same, and the foregoing optical fiber transmission line arranged therebetween.
According to the present invention, even if a power of a signal light entering an optical repeater is changed because of loss variance of the optical fiber, signal lights reach the optical receiving equipment, each having a power equal to one another.